Electronics > Beginners

Current limiting to an inductive load - do I include its resistance in calcs?

(1/2) > >>

dsegel:
I'm using a transistor to drive a coil with 18V DC. The coil has a resistance if 45 ohms. I want to limit current to about 350ma. Do I need to add a 50ohm resistor in series, or does the coil resistance already limit the current?

magic:
Of course it does. 18V/45Ω=400mA so you only need to add a little more resistance in series.

MrAl:

--- Quote from: dsegel on March 31, 2019, 08:15:44 pm ---I'm using a transistor to drive a coil with 18V DC. The coil has a resistance if 45 ohms. I want to limit current to about 350ma. Do I need to add a 50ohm resistor in series, or does the coil resistance already limit the current?

--- End quote ---

Hi,

Be prepared to catch the back EMF from the coil when you turn it off.  The voltage can shoot up very very high and cause a bad shock.
Typically a diode is used or zener or something else to absorb the energy.

dsegel:
I was planning on using something like a TVS diode because the coil is driven in both directions (i.e. the current direction could be either way). How do I choose a voltage rating? Is 30V breakdown enough for an 18V coil? I measured the voltage spike with my scope to be around +/- 150-200V.

And I can just put the diode across the coil terminals, right?

MrAl:

--- Quote from: dsegel on April 01, 2019, 04:02:08 am ---I was planning on using something like a TVS diode because the coil is driven in both directions (i.e. the current direction could be either way). How do I choose a voltage rating? Is 30V breakdown enough for an 18V coil? I measured the voltage spike with my scope to be around +/- 150-200V.

And I can just put the diode across the coil terminals, right?

--- End quote ---

Hi,

Well it partly depends on how often you have to turn on the coil and how long it stays off for, and the total energy that has to be dumped.  The energy will partly dictate the diode power rating because it has to dissipate energy.
Do you know the inductance of the coil?
And how often do you turn it on and turn it off, like the frequency?

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod